A steering control device mounted to a vehicle such as automobile serves to convert a rotation of a handle to a displacement in an axial direction of a tie rod and then to change direction of lateral wheels coupled with both end portions of the tie rod. The steering angle of the handle and the turning angle of the wheel in the steering control device generally have a constant relationship corresponding to each other. However, it is desirable to make large a rate of the turning angle with respect to the steering angle at a time of low speed driving of the automobile in a viewpoint of maneuverability of the vehicle such as easy garaging (back the car into garage).
As a steering control device for a vehicle having a conventional structure, FIG. 5 shows a steering control device utilizing a ball screw mechanism, which is disclosed in Japanese Patent Laid-open (KOKAI) Publication No. HEI 4-118382. This steering control device comprises a first ball screw mechanism 34 including a ball nut screwed to a screw shaft 32 connected to a handle 31, a second ball screw mechanism 37 including a ball nut 36 screwed to a screw shaft 35 connected to a driving wheel, and a transmission mechanism 39 coupling the ball nut 33 of the first ball screw mechanism 34 and the ball nut 36 of the second ball screw mechanism 37 together through a swingable lever member 42. The transmission mechanism 39 is provided with a connection rod 38 connecting the ball nut 33 of the first ball screw mechanism 34 to the lever member 42 in an axial direction thereof. Further, a distance between the center of swinging motion of the lever member 42 and a connecting portion of the ball nut 36 of the second ball screw mechanism 37 is adjustable.
When the handle 31 is operated, the screw shaft 32 of the first ball screw mechanism 34 is rotated and the ball nut 33 is displaced in the axial direction. According to such displacement of the ball nut 33 of the first ball screw mechanism 34, the second ball nut 36 connected to the lever member 42 of the transmission mechanism 39 is also displaced in the axial direction to thereby rotate the screw shaft 35 and transmit the steering power to the driving wheel. By adjusting, with respect to the center of the swinging motion of the lever member 42, the position of the connecting portion 41 at which the ball nut 36 of the second ball screw mechanism 37 is connected to the lever member 42, a ratio of the displacements of the respective ball nuts 33 and 36 can be changed. Accordingly, the rate of the handle steering angle with respect to the wheel turning angle can be changed.